1. Field of the Invention
The present invention relates to an email transmission method of broadcasting email including encrypted data, and relates to an email transmission device for implementing the method.
2. Description of the Related Art
In recent years, various cryptograph techniques have been carefully watched for realizing safe data communication in connection with the rapid growth of the computer network. So far, the common key cryptosystem in which an encryption key is equal to a decryption key and the public key cryptosystem in which both keys are different are widely used. Data Encryption Standards (DES) adopted by National Institute of Standards and Technology is a typical example of the common key cryptosystem, and Rivest Shamir Adleman (RSA) is a typical example of the public key cryptosystem.
The cryptosystem, which utilizes the ID (Identity) information identifying an individual such as the address, the name and the email address of each user, is also being proposed these days. In this cryptosystem, the common encryption key is generated between a sender and a receiver based on the ID information.
ID-NIKS (ID-based Non-Interactive Key Sharing Scheme) is being studied and proposed as the cryptosystem which is based on the ID information stated above and does not need the preliminary communication between a sender and a receiver prior to the cryptographic communication (that is, communication of encrypted data). Since the ID-NIKS does not need to exchange a public key and a secret key between a sender and a receiver and there is no need of the key list and the service by the third party, the communication can be established safely between any users. Furthermore, since there is no need to carry out the preliminary communication as mentioned above, there is an advantage of being convenient for users, so that this cryptosystem is expected to be a core of a cryptosystem in the future.
FIG. 6 is an explanatory drawing showing the principle of the ID-NIKS system. Assuming that there is a reliable center, the common key generating system is composed by centering this center. In FIG. 6, the ID information of an entity A is designated as the reference mark h (IDA) utilizing the hash function h (.). Regarding any entity A, the center calculates a secret key SAi based on a center public information {PCi}, a center secret information {SCi}, and the ID information h(IDA) of the entity A by the equation: SAi=Fi({SCi}, {PCi}, h(IDA)), and distributes it to the entity A.
Utilizing the secret key {SAi} of the entity A itself, the center public information {PCi} and the ID information h(IDB) of the entity B, the entity A generates a common key KAB by the equation: KAB=f({SAi}, {PCi}, h(IDB)) for encrypting and decrypting the communication established with the other optional entity B.
Moreover, in the same manner, the entity B also generates the common key KBA used between the entities A and B. If the relation that KAB is equal to KBA is constantly established, the common keys KAB, KBA can be utilized as the encryption key and the decryption key between the entities A and B.
The case of sending and receiving email utilizing the aforementioned ID-NIKS will be described. First, a sender and a receiver of email respectively acquire the secret key set based on their own email addresses (the ID information) from the center in advance. The sender generates the common key in accordance with the public key generated based on the receiver's email address (the ID information) and in accordance with the secret key acquired as described above, the data is encrypted utilizing the generated common key, and the encrypted data is sent by email. On the other hand, the receiver generates the common key in accordance with the public key generated based on the sender's email address (the ID information) and in accordance with the secret key acquired as mentioned above, and decrypts data in the received email by utilizing the common key generated as mentioned above.
In this manner, the safe sending and receiving of email can be realized easily. Additionally, the above DES etc. can be utilized in the cryptographic communication based on such ID-NIKS, for example.
When transmitting same data to a plurality of destinations by email, so-called broadcasting can be utilized. The term “broadcasting” here means the transmission method by which email can be sent to respective designated destinations by sending email having a plurality of the designated destinations only once. If such broadcasting is utilized, there is an advantage that it needs less communication volume compared with the case of sending email to the respective destinations many times.
However, when utilizing ID-NIKS, even if same data is sent to multiple destinations by email, the data must be encrypted utilizing the respective email addresses of destinations as mentioned above, and the different email must be sent to each of the destinations. Therefore, there is a problem that we cannot accomplish the advantage of the multiple-address transmission as mentioned above.
Moreover, we can not only accomplish such a advantage, but there is also a problem that it sometimes puts an enormous burden on the transmission process since the encryption process of the sending data must be executed a plurality of times as many as the number of the designated destinations.
In the meantime, as for the data communication in a computer network, a method by which data is transmitted after encrypting the data by utilizing the specified session key has been adopted in order to secure the sending data. The dishonesty such as interpolating and pretending can be prevented in advance by utilizing such a session key.